1. Field of the Invention
The present invention relates to a fixture for implantation and osseointegration with bone tissue whose purpose is to carry a prosthesis, a prosthesis anchoring system, and a prosthesis attached to the fixture.
2. Description of the Related Art
It is known to implant fixtures in a person's bone tissue in order to attach different types of prostheses to them, dental prostheses for example.
To make certain that the fixture anchorages are permanent, a number of factors must be fulfilled with regard to, among other things, material selections and operation techniques. In practice, fixtures developed by Professor Branemark made of pure titanium with micro-pitted surfaces have shown very good long-term anchoring abilities. The screw-shaped fixtures are operated into a bone, the jawbone for example, and are permitted to heal in -osseointegrate--for a certain time period, usually a few months. Thereafter, a prosthesis can be mounted on the fixtures.
Most types of these fixtures are provided with external threads--and are thus implanted screws--and they are attached by screwing them into holes made in the bone tissue. The holes are often pre-threaded, but it is also possible to screw in self-tapping screws. Normally, the fixtures require shorter or longer times for healing-in before they should be exposed to significant loads. On the fixtures, one can attach different types of prostheses, and for dental prostheses one can attach individual teeth, i.e., one tooth per fixture, but it is also possible for two or more fixtures to serve as attachment points for a bridge structure containing a number of teeth or even an entire row of teeth. The fixtures are therefore designed in a suitable way at their free ends.
The free end of such a fixture to which a prosthesis is to be fastened, i.e., the fastening end, is equipped with some form of fastening device that will engage a corresponding fastening device on the prosthesis. It is herewith usual to have the attachment arranged so that a bearing surface on the prosthesis or a prosthesis-carrying element will be in contact with a supporting surface on the fixture's fastening end. The supporting and bearing surfaces should be parallel to ensure fully satisfactory functionality. This seldom poses any problem for prostheses of the type that are attached by means of only one fixture since in such cases the prosthesis, which is usually fastened by a threaded joint to the fixture, will have its bearing surface forced into contact with the fixture's supporting surface throughout its entire area when the prosthesis is screwed into place.
In many cases, a prosthesis can be anchored using more than one fixture. This is especially true in connection with jaw reconstruction when a prosthesis containing a row of teeth is to be attached, but it can also occur for prostheses used for other parts of the body.
When such a prosthesis or a holder for such a prosthesis is to be attached to two or more fixtures, the fixture onto which the prosthesis is first attached and screwed into place will force the prosthesis into a certain position as a result of the fact that its bearing surface, which is pressed against the fixture's supporting surface will forcibly have its direction determined by said supporting surface. For screwing a prosthesis to the second fixture and to additional fixtures if any, it is desirable that the corresponding bearing and supporting surfaces on this/these fixture(s) also be parallel so that there will be no misalignment. When anchoring fixtures in bone tissue one tries, of course, to have the supporting surfaces assume their intended directions relative to each other with the greatest possible precision, and these directions should be adapted to the corresponding bearing surfaces on the prosthesis. Usually the fixture surfaces are to lie in the same plane or at least be parallel.
To fully achieve this is very difficult, and it must be expected that an anchored fixture will deviate directionally from what is intended. Anchoring two or more fixtures fully parallel is also very difficult, and the possibility of obtaining precisely matching heights is remote. A deviation of even a mere degree or so results in a deficient fit between the supporting surfaces and bearing surfaces when a prosthesis, as described above, is attached to the fixtures. Since the prosthesis element or prosthesis holder is generally made from a completely rigid material such as stainless steel, a faulty fit cannot be compensated for by deforming the element or holder. Instead, there is poor contact with one or more of the fixtures' supporting surfaces and strains develop in the prosthesis structure when one tightens the threaded joint to attach the prosthesis to the fixtures. Moreover, these strains are propagated down into the bone tissue. This can cause discomfort and trouble for the patient while detracting from the functionality of the prosthesis and shortening its life expectancy.
This problem is especially common in connection with jaw reconstruction where a superstructure with a dental prosthesis is screwed in place on the fixtures directly or via an attachment bar. Such a lack of parallelism among the fixtures poses a major problem, particularly in connection with the use of prefabricated superstructures not intended for extensive individual adaptation. In order to compensate for this non-accuracy it is earlier known e.g. from EP 0126 624, EP 0370 590 and EP 0466 267 to provide an intermediate elastic device between the fixture and the prosthesis.